The present invention relates to a method and apparatus for the autonomous identification of stars, which method or apparatus may be employed aboard a spacecraft to determine the position of that spacecraft.
The ability to identify a star, or star pattern, is becoming an increasingly important aspect of spacecraft navigation. The ability to recognize stars autonomously permits three axis orientation of spacecraft attitude to be performed, greatly enhancing the value of star camera data. Spacecraft designed with this inherent autonomy are less reliant on expensive and fragile ground communication links, more robust against system failure, and require fewer sensors. Furthermore, star identification and subsequent navigation enable higher pointing accuracy capabilities to be achieved. Autonomous star identification is of particular benefit to deep space missions where communication delays make interactive decision making inefficient and time-consuming, contributing to a significant portion of the overall mission costs.
Despite these advantages, few star camera systems with an autonomous star identification capability have been developed to date. However, one such system is described in a paper by Roelof W. H. van Bezooijen entitled "True Sky Demonstration Of An Autonomous Tracker" Vol. 2221 "Guidance, control, and tracking," SPIE, Bellingharm, Wash., 1994. The system disclosed in this paper selects pairs of stars in an image frame and determines the angular separation between each pair. This data is compared with data from database recording the angular separation of known star pairs, and a number of candidate matches are identified. This process is repeated for another pair of stars and another set of candidate matches is stored. The space co-ordinates of two sets of candidate matches are then compared and any candidates from the two sets which correspond to the same region of space are identified. A check is then performed by obtaining candidate matches for a further pair of stars within the same region and the previous candidate match confirmed if a candidate match from the third pair also corresponds to the same region of space.
The prior art system described above requires a large storage memory in which to record the angular separation distances for a great number of star pairs. Consequently a finite time is required to identify possible matches and this is sufficient to preclude real time performance.
An alternative technique is disclosed in WO 95/03214 which selects the brightest star within a received image and identifies the two nearest neighbor stars above a predeternmined threshold. The three angular separations of the triad of stars are then determined and compared with data stored in a database. With this system the quantity of data to be compared is again such as to preclude real time operation. Furthermore it is difficult to perfectly match camera intensity to the intensity used when generating the database. Even if the threshold intensity could be matched, there is a 55% chance that the nearest neighbor will not correspond to the nearest neighbor of the database (assuming star magnitudes can be determined from the images to an accuracy of .+-.0.5 magnitudes). This is because the number of stars in a reference catalog grows exponentially with catalog size and therefore many stars always approach the cut off intensity no matter where that cut off intensity level is set. The intensity will vary slightly, due to shifts in spectrum sensitivity of a star camera and other efforts associated with discrete sampling. Because 55% of the stars in the image may not be in the database, different nearest neighbors will be identified in the image relative to those identified when the database was generated.
It is an object of the present invention to provide an improved method and apparatus for identifying a star, which method and apparatus aim to overcome the problems identified with the systems described above.